It doesn't need cgroups or containers, because every process has its own namespace, its own view of the network-global filesystem, so everything is in a container by default.
It doesn't need a microkernel, because the concept of microkernels is to split a big monolithic kernel into lots of small simple "servers" running in user space, and have them communicate by passing messages over a defined communications protocol, some kind of RPC type thing. It works, and QNX is the existence proof. But it's really hard and it's really inefficient -- of which, the HURD and Minix 3 are the existence proofs.
So most of the actual working "microkernel" OSes kludge it by embedding a huge in-kernel "Unix server" that negates the entire microkernel concept but delivers compatibility and performance. Apple macOS and iOS are the existence proof here. (It could be argued that Windows NT 4 and later are also examples.)
Plan 9 achieves the same result, without the difficulties, by default by having most things user space processes and communicating via the filesystem.
Disclaimer: this is my very rudimentary understanding. I am not an expert on Plan 9 by any means.
Cgroups are a lot more than just "namespaces". It is also the mechanism by which you can constrain how much CPU, Memory, Network Bandwidth, Storage IOPS or Throughput, etc., processes in a particular cgroup or container can use.
But I think the core point here is that, as with much of the Plan 9 design, by including a more elegant and powerful abstraction in the core design, the need for a much more powerful and much more complicated abstraction layer on was obviated, if not eliminated.
It doesn't need a microkernel, because the concept of microkernels is to split a big monolithic kernel into lots of small simple "servers" running in user space, and have them communicate by passing messages over a defined communications protocol, some kind of RPC type thing. It works, and QNX is the existence proof. But it's really hard and it's really inefficient -- of which, the HURD and Minix 3 are the existence proofs.
So most of the actual working "microkernel" OSes kludge it by embedding a huge in-kernel "Unix server" that negates the entire microkernel concept but delivers compatibility and performance. Apple macOS and iOS are the existence proof here. (It could be argued that Windows NT 4 and later are also examples.)
Plan 9 achieves the same result, without the difficulties, by default by having most things user space processes and communicating via the filesystem.
Disclaimer: this is my very rudimentary understanding. I am not an expert on Plan 9 by any means.